1. Field of the Invention
This invention relates to pulse code modulation telephone switching systems generally and more particularly, to an arrangement for flexibly intermixing digital and analog trunks.
2. Description of the Prior Art
Private automatic branch telephone exchanges function as centralized switching systems providing connection between a number of locally connected telephone lines with associated telephone apparatus and one or more trunk circuits connecting the private automatic branch exchange to a distant central office.
Until very recent times private automatic branch exchanges (PABX's) have provided switching between lines and trunks on a space divided basis. That is, switches of either an electro-mechanical or electronic configuration have provided selective paths through the switching system to interconnect lines to each other or to trunk circuits serving the PABX. In such systems the signals transmitted through the PABX were generally of an analog nature. Where a line or a trunk circuit employing digital techniques such as pulse code modulation was employed, interface circuitry providing analog to digital conversion and digital to analog conversion were a necessity.
More recently a new generation of PABX systems employing time division switching have been provided. Some such systems as the "Dimension" PABX manufactured by Western Electric Company have provided time division switching of analog signals. Other more recent developments in PABX systems have provided time division switching of pulse code modulated signals. Systems of this type have been manufactured by GTE Automatic Electric Company and designated GTD 120, GTD 1000 and GTD 4600. In such systems as the GTD series, analog to digital, digital to analog interfaces have been provided between the lines and trunks and the time division switching system.
To effect greater economies in transmission equipment more extensive use has been made in recent years of digital transmission equipment. Of particular wide acceptance has been the so called T1 type carrier systems which employ pulse code modulation to provide a number of multiplexed signal paths over a single transmission facility such arrangements are currently in use primarily between telephone central offices. To date little utilization of such economies has taken place in transmission facilities between central offices and private automatic branch exchanges. The state of the art and time division switching systems employing pulse code modulated signals as the transmission format is exemplified by such systems as the aforementioned GTD 120 the operation of which is described in U.S. Pat. No. 4,007,338 issued to D. W. McLaughlin. The use of two one way lines for signalling in the D2 or D3 PCM type format is discussed in the article "Second Generation Toll Quality PCM Carrier Terminal" by L. Dean Crawford in the April, 1972 issue of the Automatic Electric Technical Journal. A channel bank unit of the type employed and as described above is manufactured by GTE Lenkurt Incorporated and designated the 9002A channel bank.
Accordingly, it is the primary object of this invention to provide facilities in a private branch exchange for trunk circuits connected via a T1 span line and employing pulse code modulation without the introduction of channel bank equipment and to be able to use the fixed trunk numbering format of the PABX.